8 research outputs found
A New Era in the Quest for Dark Matter
There is a growing sense of `crisis' in the dark matter community, due to the
absence of evidence for the most popular candidates such as weakly interacting
massive particles, axions, and sterile neutrinos, despite the enormous effort
that has gone into searching for these particles. Here, we discuss what we have
learned about the nature of dark matter from past experiments, and the
implications for planned dark matter searches in the next decade. We argue that
diversifying the experimental effort, incorporating astronomical surveys and
gravitational wave observations, is our best hope to make progress on the dark
matter problem.Comment: Published in Nature, online on 04 Oct 2018. 13 pages, 1 figur
Formation of Supermassive Black Holes
Evidence shows that massive black holes reside in most local galaxies.
Studies have also established a number of relations between the MBH mass and
properties of the host galaxy such as bulge mass and velocity dispersion. These
results suggest that central MBHs, while much less massive than the host (~
0.1%), are linked to the evolution of galactic structure. In hierarchical
cosmologies, a single big galaxy today can be traced back to the stage when it
was split up in hundreds of smaller components. Did MBH seeds form with the
same efficiency in small proto-galaxies, or did their formation had to await
the buildup of substantial galaxies with deeper potential wells? I briefly
review here some of the physical processes that are conducive to the evolution
of the massive black hole population. I will discuss black hole formation
processes for `seed' black holes that are likely to place at early cosmic
epochs, and possible observational tests of these scenarios.Comment: To appear in The Astronomy and Astrophysics Review. The final
publication is available at http://www.springerlink.co
The Formation of the First Massive Black Holes
Supermassive black holes (SMBHs) are common in local galactic nuclei, and
SMBHs as massive as several billion solar masses already exist at redshift z=6.
These earliest SMBHs may grow by the combination of radiation-pressure-limited
accretion and mergers of stellar-mass seed BHs, left behind by the first
generation of metal-free stars, or may be formed by more rapid direct collapse
of gas in rare special environments where dense gas can accumulate without
first fragmenting into stars. This chapter offers a review of these two
competing scenarios, as well as some more exotic alternative ideas. It also
briefly discusses how the different models may be distinguished in the future
by observations with JWST, (e)LISA and other instruments.Comment: 47 pages with 306 references; this review is a chapter in "The First
Galaxies - Theoretical Predictions and Observational Clues", Springer
Astrophysics and Space Science Library, Eds. T. Wiklind, V. Bromm & B.
Mobasher, in pres
Prospects for detecting supersymmetric dark matter in the Galactic halo.
Dark matter is the dominant form of matter in the universe, but its nature is
unknown. It is plausibly an elementary particle, perhaps the lightest
supersymmetric partner of known particle species. In this case, annihilation of
dark matter in the halo of the Milky Way should produce gamma-rays at a level
which may soon be observable. Previous work has argued that the annihilation
signal will be dominated by emission from very small clumps (perhaps smaller
even than the Earth) which would be most easily detected where they cluster
together in the dark matter halos of dwarf satellite galaxies. Here we show,
using the largest ever simulation of the formation of a galactic halo, that
such small-scale structure will, in fact, have a negligible impact on dark
matter detectability. Rather, the dominant and likely most easily detectable
signal will be produced by diffuse dark matter in the main halo of the Milky
Way. If the main halo is strongly detected, then small dark matter clumps
should also be visible, but may well contain no stars, thereby confirming a key
prediction of the Cold Dark Matter (CDM) model.Comment: to appear in Nature, 23 pages, 8 figures, includes Supplementary
Informatio
Cosmology with the Laser Interferometer Space Antenna
The Laser Interferometer Space Antenna (LISA) has two scientific objectives of cosmological focus: to probe the expansion rate of the universe, and to understand stochastic gravitational-wave backgrounds and their implications for early universe and particle physics, from the MeV to the Planck scale. However, the range of potential cosmological applications of gravitational wave observations extends well beyond these two objectives. This publication presents a summary of the state of the art in LISA cosmology, theory and methods, and identifies new opportunities to use gravitational wave observations by LISA to probe the universe